1. Technical Field
The present invention relates to a sputtering target for an oxide semi-conductor, comprising InGaO3(ZnO) crystal phase and a process for producing thereof, as well as a forming of an oxide semi-conductor thin film or a thin film transistor using the sputtering target.
2. Background of the Art
A field effect type transistor is widely used for a unit electron device for a semi-conductor memory integrated circuit, a high-frequency signal amplifier device, a device for driving a liquid crystal, and is the most practically used electric device.
Among the above, because of recent dramatic progress in a display device, a thin film transistor (TFT) is heavily used as a switching device for driving a display device by adding drive voltage to a display device among various kind of a display device such as a liquid crystal display device (LCD), an electroluminescence display device (EL), or a field emission display (FED).
As for the material thereof, silicon semi-conductor compounds are most commonly used. In general, single-crystal silicon is used for a high-frequency signal amplifier device or an element for integrated circuit which requires high-speed operation. Amorphous silicon is used for a device for driving a liquid crystal to meet a request for a larger-area.
However, a crystalline silicon thin film requires, for example, high temperature of 800° C. or more during crystallization, and thus it is difficult to compose it on a glass substrate or an organic material substrate. Therefore, there are several problems, for example, the film can be formed only on an expensive substrate having high heat resistance such as a silicon wafer or quartz, and the production thereof requires a lot of energy and many steps.
In addition, it was difficult for a crystalline silicon semi-conductor to uniformity make a large area semi-conductor to be applied to a big screen TV, and to reduce costs by cutting a number of masks down.
An amorphous silicon semi-conductor capable of forming under relatively low temperature (amorphous silicon) has lower mobility and slower switching speed than those of crystalline semi-conductor. Therefore, when the semi-conductor is used for a switching device for driving a display device, displaying high-speed motion picture cannot be achieved sometimes.
Currently, a switching device for driving a display device mainly uses a device using a silicon semi-conductor film. This is because, a silicon thin film has various good properties such as stability, good workability, and high switching speed. This silicon thin film is generally prepared by a chemical vapor deposition (CVD) method.
For example, a conventional thin film transistor (TFT) has an opposite stagger structure which laminates a gate electrode, a gate insulation layer, a semi-conductor layer such as a hydrogenated amorphous silicon (a-Si:H), a source electrode and drain electrode on a substrate such as a glass. This TFT is used for an image sensor, as well as a driving device for a flat panel display represented by an active matrix type liquid crystal display, within a field of a large-area device. Among these usages, high mobility is required for higher definition and higher speed in driving frequency, even though conventional amorphous silicon is used.
Under these situations, recently, an oxide semi-conductor thin film using oxides is coming up, because it is to make a large area and to reduce costs by cutting a number of masks down rather than using crystalline silicon semi-conductor, and the film has excellent stability and high mobility rather than those of an amorphous silicon semi-conductor (amorphous silicon).
However, among those types of oxide semi-conductors, an oxide semi-conductor using zinc oxide and a multi-crystalline film has low field effect mobility (hereinafter, simply said as “mobility”): about 1 cm2/V·sec, and low ON-OFF ratio. In addition, leak current is easy to occur, and thus it was difficult to put it into industrial, practical use. Furthermore, many attempts were taken place for an oxide semi-conductor using zinc oxide, but the following problems were present when a film was formed by a sputtering method which is commercially and commonly used.
That is, an oxide semi-conductor film containing ZnO as a major component easily causes oxygen defect, many occurrences of carrier electrons, and difficulties in lessen electric conductivity. During film forming by using a sputtering method, abnormal discharges were occurred, stability for forming films is lost, and thus uniformity and reproducibility of the obtained film were declined. Therefore, for, example, when it is used as an active layer (channel layer) of a TFT (thin film transistor), even if no gate voltage is added, a lot of current is passed between a source terminal and a drain terminal, thus normally off of a TFT cannot be achieved. It is difficult to enlarge ON-OFF ratio of a transistor. Further, there were possibility that mobility is low, ON-OFF ratio is low, leak current is high, pinch-off is unclear, normally-ON is easy to occur, and thus properties in TFT are lowered. The obtained film has less chemical resistance, and thus process or usage environment is limited, for example, wet-etching is difficult. Further, a film must be formed under high pressure in order to improve its properties, but film-forming speed is slow as well as 700° C. or more of high temperature is required, and thus there are problems in industrialization. In addition, TFT properties such as mobility in a bottom-gate structure is low, and thus a TFT device structure is also limited, for example, top-gate structure and 200 nm or more of thickness are required for raising its properties.
In order to solve these problems, a method for producing an amorphous oxide semi-conductor film composed of indium oxide, gallium oxide and zinc oxide so as to drive a thin film transistor has been discussed. Further, there was discussion for industrially preparing an amorphous oxide semi-conductor comprising indium oxide, gallium oxide and zinc oxide by a sputtering method which is suitable for commercial production. Sputtering targets for the method were also discussed. For example, a sputtering target comprising a homologous structure of InGaZnO4 (InGaO3(ZnO)) in which the content of Ga in atom ratio is the same as that of In is disclosed (Patent Document 1).
In addition, a sputtering target only having a homologous structure of InGaMgO4 and a sputtering target only having a homologous structure of YbFe2O4 is also disclosed (Patent Document 2). However, if a sputtering target composed of oxides comprising In, Ga, Zn is film-formed by using methods disclosed in these documents, problems were raised, e.g. the content ratio of In and Ga in atom ratio is shifted between a semi-conductor film and the sputtering target, properties of a thin film transistor was changed during long term film forming (decline in reproducibility), film-forming speed was changed during continuous film forming. In addition, it was reported that crystal of InGaO3(ZnO) was obtained by sintering very small amount of a mix powder fin a platinum tube at each temperature (1150, 1250, 1350, 1550° C.) for a long time (3 to 14 days) (Non-Patent Documents 1 and 2). However, it is difficult to form a film using thus obtained crystalline powder, and thus no production method for a sputtering target which is capable of film-forming by a sputtering method was discussed.
Meanwhile, there was a discussion about an effect cased by inhibiting the formation of a Ga2O3 crystalline phase having high insulation (Patent Document 3). However, there was no discussion about effects by a crystalline phase where a peak between 2θ=62 degree and 63 degree based on X-ray diffraction such as a ZnGa2O4 crystalline phase, a In2O3 crystalline phase, a ZnO crystalline phase, a In2O3(ZnO)3 crystalline phase, and a InGaO3 crystalline phase.
At the same time, as an embodiment of a composition having Ga content which is smaller than In content in atom ratio, an example for forming an amorphous oxide semi-conductor and a thin film transistor using an In—Ga—Zn—O sintered body having metal ratio of In:Ga:Zn=30:15:55 are disclosed (Patent Document 4). If this sintered body disclosed in this document is used as a sputtering target, several problems are occurred, for example, shifting composition ratio of In and Ga in atom ratio between a film-formed semi-conductor film and the sputtering target, low reproducibility of a thin film transistor, occurring variation during forming a thin film transistor to its large area form, and a lot of particles being appeared. Thus there was obstacle in practical use.
As for another embodiment of a composition having Ga content which is smaller than In content in atom ratio, a sputtering target comprising ZnO as a main component. However, because of ZnO being a main component, a crystalline phase other than InGaO3(ZnO) was formed, density of a target becomes low, resistance becomes high, and acid-tolerance at the making of a semi-conductor film becomes low. Therefore, the target was not suitable for a field effect transistor (Patent Documents 5 and 6).
As stated above, a conventional target comprising In, Ga and Zn includes a crystalline phase other than InGaO3(ZnO). Therefore, composition ratio readily shifts between a target and a film-formed semiconductor film, bulk resistance was high, uniformity was low, variation of film-forming speed during the film-forming was high, and many particles were appeared. When such a target was used for film-forming of a semi-conductor, there were problems about appearance of variation when it was prepared to its large area form.    Patent Document 1: JP-A-2007-73312    Patent Document 2: JP-B-3947575    Patent Document 3: JP-A-2007-223849    Patent Document 4: JP-A-2008-53356    Patent Document 5: WO2004/079038    Patent Document 6: JP-B-3644647    Non-Patent Document 1: M. Nakamura, N. Kimizuka and T. Mohri, J. Solid State Chem, vol. 93, No. 2, page 298, (1991)Non-Patent Document 2: M. Nakamura, et. al., J. Solid State Chem, vol. 116, No. 2, page 170, (1995)